1. Field of the Invention
The present invention relates to a structure of a lock nut and components thereof, consisting of three nuts combined to be screwed onto a bolt for anti-loosening. Of the three nuts the outermost nut is the same as the ordinary nut widely available in the market.
2. Description of the Related Arts
As measures of stopping loosening by a plurality of nuts, use has been hitherto made of a double nut including a couple of nuts having the same shape. After firmly screwing one nut onto a material to be fastened, the other nut is fastened to the one nut.
The double nut proposed in Japanese Patent Publication No. 60-249718 was an improvement of the conventional double nut.
Such conventional double nut was subjected to a frequent loosening compared to a single nut. As shown in FIG. 8, this arises from the fact the female thread surface of the upper (outer) nut 18 and the surface of the male thread 17 of the bolt 16 are only substantially brought into contact with each other, whereas the female thread surface of the lower nut 19 and the surface of the male thread of the bolt do not substantially come into contact with each other.
The following is the detailed description of this conventional double nut. As shown in FIG. 10, a nut 18 is fastened to the top surface of another nut 19. By fastening the nut 19 onto the male thread 17, shown in FIG. 8, the shank of the male thread 17 is upwardly pulled to bring only the upper surface side of the male thread of the nut 19 into press-contact with the lower surface of the male thread 17. When the outer nut 18 is fastened to the inner nut 19 under such conditions as shown in FIG. 10, the shank of the bolt 16 is upwardly pulled by the outer nut 18. This causes the contact surfaces of the male thread of the inner nut 19 and male thread 17 of the bolt 16 to separate from each other.
As a result, the male thread 17 and the female thread of the inner nut 19 are not allowed to be in contact with each other, thereby making it impossible to utilize the frictional resistance between the two nuts to prevent loosening. Therefore, the inner nut 19 easily rotates to cancel the anti-loosening effect.
It will be understood that in the double nut structure shown in FIG. 9 two nuts are fastened to the male thread 17 of the bolt 16 without the member to be fastened coming into contact with the inner nut 19, and hence this double nut structure has an extremely strong locking effect. The reason is that since the bolt 16 is pulled by the outer nut 18 and the inner nut 19 in the opposite direction and the two nuts are displaceable in the repellent directions, the outer nut 18 and the inner nut are brought into contact with the male thread 17.
On the contrary, when a pressing force is applied on the upper and lower surfaces of the inner nut 19, the inner nut 19 is not permitted to be displaced in any directions, and hence the male tread of the inner nut 19 and the male thread 17 of the bolt 16 are not substantially brought into contact with each other. It will be obvious that as a result of this, the anti-loosening effect is impaired. The same applies to the double nut proposed in Japanese Patent Publication 60-249718.